A process for determining the exhaust gas temperature is known from DE 38 35 852 A1, whereby the internal resistance of a lambda sensor positioned in the exhaust gas is measured. Under stationary conditions, the exhaust gas temperature (T.sub.a) can be inferred on the basis of our knowledge of the relationships between internal resistance (R.sub.i) and probe temperature (T.sub.s). DE 38 35 852 A1 provides additional options to account for the effects of the composition of the mixture on the accuracy of the temperature reading. The process which is known as a result of this state of the art operates at a degree of precision of 0.5 per cent. According to DE 38 35 852 A1, a multitude of internal resistance temperature characteristic curves, each for a different lambda value, is needed to achieve more precise readings. Furthermore, the operating conditions are factored into the arithmetic relationship between T.sub.s and T.sub.a.
DE 43 39 692 A1 discloses a generic process for determining the exhaust temperature by means of an electrically heated lambda probe, where the exhaust gas temperature is inferred by the amount of electrical power needed to keep the lambda probe at a constant temperature. In this process, the exhaust gas temperature can be determined directly if the flow of exhaust gas remains virtually constant. However, if the exhaust gas flow rate changes together with the operating condition of the combustion system, a characteristic curve must be plotted for each individual operating condition, with the composite view of these curves producing a characteristic diagram. Other parameters that influence the operating condition and, consequently, the measuring inaccuracy of the sensors, must also be taken into account when determining the exhaust gas temperature. For example, because of the relationship between engine speed and exhaust gas flow rate, a characteristic diagram involving electric output and engine speed must also be used to determine the exhaust gas temperature. Other possible parameters include the throttle angle or the manifold pressure. Given the multitude of possible influencing parameters, this process cannot be expected to deliver a substantial degree of accuracy.
The combination of a heated lambda probe with variable sensor characteristics and an additional lambda probe with constant characteristics is described in DE 43 20 881. Because of the spatial proximity of the two sensor elements, the constant signal can be calibrated by means of the variable signal, provided the probe temperature is known. The additional electron-conducting temperature sensor is used to measure the probe temperature, so that a constant temperature level can be maintained. The calculation of the exhaust gas temperature, as described in the processes according to DE 38 35 852 A1 and DE 43 39 692 A1, is not described here. The temperature-sensitive element considered in DE 38 35 852 A1 and DE 43 39 692 A1 is the internal resistance of the ion-conducting electrolytes. However, the internal resistance of the electrolytes only enters the range of measurable resistance at higher temperatures. Testing of exhaust gas in the range of 0 to 250.degree. C. is not possible.
Due to polarization phenomena, the measurement of internal resistance is associated with substantial metrological complexity. The lambda probes described there are highly unfavorable in terms of their geometry. Because of the low immersion depth of the tailpipe, only peripheral flow is measured and the reading is heavily falsified by the temperature of the tailpipe.
The objective of this invention is to provide a process and a suitable sensor arrangement for the determination of the exhaust gas temperature and of lambda that permits a precise temperature measurement across the entire temperature range and under various operating conditions, especially in non-stationary cases, and with minimal metrological complexity.